Kirill N. Alekseev

Terahertz Bloch oscillator with a modulated bias.

Electrons performing Bloch oscillations in an energy band of a dc-biased superlattice in the presence of weak dissipation can potentially generate THz fields at room temperature. The realization of such a Bloch oscillator is a long-standing problem due to the instability of a homogeneous electric field in conditions of negative differential conductivity. We establish the theoretical feasibility of stable THz gain in a long superlattice device in which the bias is quasistatically modulated by microwave fields. The modulation waveforms must have at least two harmonics in their spectra.

Effect of temperature on resonant electron transport through stochastic conduction channels in superlattices

We show that resonant electron transport in semiconductor superlattices with an applied electric and tilted magnetic field can, surprisingly, become more pronounced as the lattice and conduction electron temperature increases from 4.2 K to room temperature and beyond. It has previously been demonstrated that at certain critical field parameters, the semiclassical trajectories of electrons in the lowest miniband of the superlattice change abruptly from fully localized to completely unbounded. The unbounded electron orbits propagate through intricate web patterns, known as stochastic webs, in phase space, which act as conduction channels for the electrons and produce a series of resonant peaks in the electron drift velocity versus electric-field curves. Here, we show that increasing the lattice temperature strengthens these resonant peaks due to a subtle interplay between the thermal population of the conduction channels and transport along them. This enhances both the electron drift velocity and the influence of the stochastic webs on the current-voltage characteristics, which we calculate by making self-consistent solutions of the coupled electron transport and Poisson equations throughout the superlattice. These solutions reveal that increasing the temperature also transforms the collective electron dynamics by changing both the threshold voltage required for the onset of self-sustained current oscillations, produced by propagating charge domains, and the oscillation frequency.

Dissipative Chaos in Semiconductor Superlattices

We consider the motion of ballistic electrons in a miniband of a semiconductor superlattice (SSL) under the influence of an external, time-periodic electric field. We use a semiclassical, balance-equation approach, which incorporates elastic and inelastic scattering (as dissipation) and the self-consistent field generated by the electron motion. The coupling of electrons in the miniband to the self-consistent field produces a cooperative nonlinear oscillatory mode which, when interacting with the oscillatory external field and the intrinsic Bloch-type oscillatory mode, can lead to complicated dynamics, including dissipative chaos. For a range of values of the dissipation parameters we determine the regions in the amplitude-frequency plane of the external field in which chaos can occur. Our results suggest that for terahertz external fields of the amplitudes achieved by present-day free-electron lasers, chaos may be observable in SSL{close_quote}s. We clarify the nature of this interesting nonlinear dynamics in the superlattice{endash}external-field system by exploring analogies to the Dicke model of an ensemble of two-level atoms coupled with a resonant cavity field, and to Josephson junctions. {copyright} {ital 1996 The American Physical Society.}

Bloch gain in dc-ac-driven semiconductor superlattices in the absence of electric domains

We theoretically study the feasibility of amplification and generation of terahertz radiation in dc-ac-driven semiconductor superlattices in the absence of electric domains. We find that if in addition to a dc bias a strong terahertz pump field is applied, a Bloch gain profile for a small terahertz signal can be achieved under the conditions of a positive static differential conductivity. Here, the positive differential conductivity arises, similarly to the case of a large-signal amplification scheme [H. Kroemer, arXiv:cond-mat/0009311 (unpublished)], due to modifications in dc current density caused by the application of a high-frequency ac field [ K. Unterrainer et al. Phys. Rev. Lett. 76 2973 (1996)]. Whereas the sign of absorption at low and zero frequencies is sensitive to the ac fields, the gain profile in the vicinity of the gain maximum is robust. We suggest to use this ac-induced effect in a starter for a terahertz Bloch oscillator. Our analysis demonstrates that the application of a short terahertz pulse to a superlattice allows the suppression of the undesirable formation of electric domains and the achievement of a sustained large-amplitude operation of the dc-biased Bloch oscillator.

Direct-current generation due to wave mixing in semiconductors

We describe an effect of the generation of direct current which may arise in semiconductors or semiconductor microstructures due to a mixing of coherent electromagnetic radiations of commensurate frequencies. The effect is, in essence, due to a nonparabolicity of the electron energy bands and is stronger in systems where this nonparabolicity is greater. We have made exact calculations in the framework of the Kane model, applicable to narrow-gap semiconductors and the tight-binding model which we employ for a description of a semiconductor superlattice.

Spontaneous dc Current Generation in a Resistively Shunted Semiconductor Superlattice Driven by a Terahertz Field

We study a resistively shunted semiconductor superlattice subject to a high-frequency electric field. Using a balance equation approach that incorporates the influence of the electric circuit, we determine numerically a range of amplitude and frequency of the ac field for which a dc bias and current are generated spontaneously and show that this region is likely accessible to current experiments. Our simulations reveal that the Bloch frequency corresponding to the spontaneous dc bias is approximately an integer multiple of the ac field frequency.

Model of the Influence of an External Magnetic Field on the Gain of Terahertz Radiation from Semiconductor Superlattices

We theoretically analyze the influence of magnetic field on small-signal absorption and gain in a superlattice. We predict a very large and tunable THz gain due to nonlinear cyclotron oscillations in crossed electric and magnetic fields. In contrast to Bloch gain, here the superlattice is in an electrically stable state. We also find that THz Bloch gain can be significantly enhanced with a perpendicular magnetic field. If the magnetic field is tilted with respect to the superlattice axis, the usually unstable Bloch gain profile becomes stable in the vicinity of Stark-cyclotron resonances.

Pendulum limit, chaos and phase-locking in the dynamics of ac-driven semiconductor superlattices

Abstract We describe a limiting case when nonlinear dynamics of an ac-driven semiconductor superlattice in the miniband transport regime is governed by a periodically forced and damped pendulum equations. We find analytically the conditions for a transition to chaos. With increasing temperature the chaos disappears. We also discuss fractional dc voltage states in a superlattice originating from phase-locked states of the pendulum.

Terahertz parametric gain in semiconductor superlattices in the absence of electric domains

The authors theoretically show that conditions for terahertz gain and conditions for formation of destructive electric domains in semiconductor superlattices are fairly different in the case of parametric generation and amplification. Action of an unbiased high-frequency electric field on a superlattice causes a periodic variation of energy and effective mass of miniband electrons. This parametric effect can result in a significant gain at some even harmonic of the pump frequency without the formation of electric domains and corruption from pump harmonics.

Dynamical instabilities and deterministic chaos in ballistic electron motion in semiconductor superlattices

Abstract We consider the motion of ballistic electrons within a superlattice miniband under the influence of an alternating electric field. We show that the interaction of electrons with the self-consistent electromagnetic field generated by the electron current may lead to the transition from regular to chaotic dynamics. We estimate the conditions for the experimental observation of this deterministic chaos and discuss the similarities of the superlattice system with the other condensed matter and quantum optical systems.